When you're talking to someone older/with more respect than you, I know you normally need to say "desu" a lot. But after everything? Like can you just say "Hontou ni?" or "Doushite?" or something just by itself, or do you always need desu?So...is there ever too much desu?

I would recommend against "maji desu ka" since "maji" is a slang word to begin with. Sometimes kids will say "majissu ka" but that's not really something to emulate, IMO.

But yes, in general, you always need to end sentences with masu forms when speaking politely to someone, whether you are tired of it or not. Often native Japanese will end sentences with -te forms or "kedo..." even in polite speech so they actually don't end up using "desu" as much as you might think.

And bringing up "hontou ni desu ka?" do you need the "ni" when it's only desu? I thought you didn't use it then.

This is an example of the widening use of "desu" as a general politeness marker that can be added to virtually anything. I think that "hontou ni desu ka" is still not widespread enough to recommend its use, though.

When you're talking to your parents/grandparents/etc., and you're really close to them, do you still need to use formal speech just because of their age/status? And what if you have a really close friend who's twice your age or something?

When you're talking to your parents/grandparents/etc., and you're really close to them, do you still need to use formal speech just because of their age/status? And what if you have a really close friend who's twice your age or something?

AS for parents, none of the kids I ran into would be extra polite to their parents. It was a mix of plain and polite forms. most conversation was plain form. I imagine that it might change as the children get older. Those that I saw were from the ages of 3-14 or so.

I just don't know that there is a "hard" rule and I imagine that each family has their own set of do's and don'ts when it comes to "keigo."